Most likely aphids. Loads of them! Either squish or hose off, then repeat as needed.

Yarrows have had a history as both food and medicine and a companion plant to ward off insect pests, but if grown in a soil that is too good it will, like any other plant, attract insect pests such as the Aphids. Like anyother plant the simplest way to get rid of those Aphids is with a sharp stream of water, but it is not necessary to hose them off since the water pressure from your garden hose could do more damage then the Aphids. A hand held sprayer produces a sharp water stream that wil knock the wee buggers off your plants.

I've never damaged a plant by spraying it with water from the hose, kimmsr. You need to reduce your water pressure or invest in different nozzle if you are having that kind of problem. I actually use my thumb to adjust the kind of spray I need, something that you might try.

You make an invalid assumption Rhizo because I have never damaged a plant with my hose either. But numerous people have written in to these forums and stated that they have.